More Labour MPs have said they will rebel against an order from party leader Jeremy Corbyn to back a bill that will trigger the Brexit process.
Party whip Jeff Smith has said he will defy a three-line whip on the EU (Notification of Withdrawal) Bill.
Shadow ministers Daniel Zeichner and Tulip Siddiq will vote against it. Ms Siddiq quit the front bench over it.
Shadow home secretary Diane Abbott said voting against the bill would “be very undermining of democracy”.
“MPs voted for a referendum, there was an extraordinary high turn out – 72% – 17m people voted to leave. Many of them in some of our poorest areas,” she told the BBC.
“How would it look if a bunch of politicians and commentators in London turned round and said: ‘We know you voted to leave but we’re just going to ignore you?'”
The European Union (Notification of Withdrawal) Bill was produced after the Supreme Court ruled that Parliament – not just the government alone – must vote to trigger Article 50 of the Lisbon Treaty, which starts the formal process of the UK leaving the EU.
Labour leader Mr Corbyn is to impose a three-line whip, ordering his MPs to back the bill. He said he understood the pressures faced – many Labour MPs represent constituencies which voted to remain in the European Union – but called on them to “unite” around “important issues”.
Frontbench members of parties are generally expected to resign from their post if they decided to defy a three-line whip.
Manchester Withington MP Jeff Smith told the Manchester Evening News: “My constituents voted strongly for remain and I think it’s important to represent their view.”
Another whip, Bristol West MP Thangam Debbonaire, told the Bristol Post she was “minded to vote against” the bill to represent her Remain-supporting constituency, but added: “It will be a tough decision.”
Meanwhile, senior Labour backbencher Meg Hillier told the BBC some of her east London constituents were “horrified” at Mr Corbyn’s stance.
“Certainly in Hackney the rage in the room was palpable – and people are really concerned. My constituency voted 78% to remain [in the EU] and while a lot of those people recognise the outcome of the referendum, we just don’t want a blank cheque.”
Tulip Siddiq quit as shadow early years minister on Thursday, saying she “cannot reconcile myself to the front-bench position”.
Mr Zeichner, shadow transport minister, said he would defy the whip and vote against the bill. “It’s my strongly held personal position, and I represent three-quarters of the people of Cambridge,” he told the Cambridge News.
“I’ve had perfectly civilised conversations (with the Labour leadership). They know my position and they understand exactly why I’m doing what I’m doing and it’s for them to decide what to do next.”
Prime Minister Theresa May has promised to begin the formal process of quitting the European Union by the end of March.
The bill is due to be initially debated by MPs on Tuesday – in a sitting that may last until midnight – and clear the Commons on 8 February, after which it will move to the House of Lords.
The Liberal Democrats have vowed to oppose Article 50 unless there is a guarantee of another referendum on the final Brexit deal that is agreed with Brussels, while the SNP has vowed to table 50 amendments to the legislation.